1. Field of the Invention
The present invention relates to an intake air amount control apparatus for an engine, which is provided with a variable valve unit used for moving an intake valve while variably changing the opening characteristic of the intake valve, and an electronically controlled throttle disposed in an intake pipe on the upstream side of the intake valve. The present invention also relates to a control method of an intake air amount of an engine.
2. Description of the Related Art
Japanese Laid-open (Kokai) Patent Application Publication No. 2001-173470 discloses an engine control apparatus which is configured to control an angle of opening of an electronically controlled throttle (it will hereinafter be abbreviated as “opening angle of the electronically controlled throttle”), based on a target boost pressure of an engine and then controls a timing of closing (it will hereinafter be abbreviated as a closing timing) of the intake valve based on a target intake air amount.
The control apparatus operates so as to correct either the target intake air amount for calculating the closing timing of the intake valve or the closing timing of the intake valve, which has been calculated based on the target intake air amount, in response to the above-mentioned target boost pressure.
With this conventional engine control apparatus, as described above, when the opening angle of the electronically controlled throttle is feed-forward controlled based on the target boost pressure while controlling the closing timing of the intake valve based on the target intake air amount, if the atmospheric pressure changes, there will occur a difference between the actually obtained boost pressure and the target boost pressure.
On the other hand, as an alternative controlling method, if an absolute pressure sensor capable of detecting an absolute pressure of the boost pressure is employed, and if a boost pressure detected by the absolute pressure sensor is brought into coincidence with the target boost pressure by controlling the opening angle of the electronically controlled throttle in the feed-back control manner, it is possible to converge the boost pressure expressed by the absolute pressure to a pressure in the quite vicinity of the target boost pressure.
However, if the atmospheric pressure decreases, a pressure detected by the absolute pressure sensor also decreases. Consequently, control operation for increasing the opening angle of the electronically controlled throttle will be made, resulting in generation of a lesser differential pressure (a negative pressure) between the atmospheric pressure and a pressure appearing in the upstream side of the intake valve in response to such a decrease in the atmospheric pressure per se.
If the negative pressure (the differential pressure) is lessened due to a decrease in the atmospheric pressure as described above, all kinds of apparatus and mechanism utilizing the negative pressure (differential pressure) in the system of an intake pipe might fail to exhibit a prescribed operation and performance, thereby possibly worsening the exhaust emission from an engine or degrading a driving performance of the engine.
In addition, if reduction in the boost pressure is conducted by reducing the opening angle of the electronically controlled throttle for the purpose of preventing the negative pressure (the differential pressure) from lessening in response to the decrease in the atmospheric pressure, there arises a problem such that a difficulty is encountered in reliable acquirement of the target intake air amount.